Semiconductor device and method for manufacturing the same

ABSTRACT

It is made possible to provide a method for manufacturing a semiconductor device that has a high-quality insulating film in which defects are not easily formed, and experiences less leakage current. A method for manufacturing a semiconductor device, includes: forming an amorphous silicon layer on an insulating layer; introducing oxygen into the amorphous silicon layer; and forming a silicon oxynitride layer by nitriding the amorphous silicon layer having oxygen introduced thereinto.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2008-11308 filed on Jan. 22, 2008in Japan, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

2. Related Art

Flash memories that are nonvolatile memories, especially NAND flashmemories, have been vigorously developed as still-image recording mediaand high-quality audio recording media, since such flash memories can bereadily made smaller in size, and can be provided with a large amount ofmemory at low costs. Furthermore, such flash memories areshock-resistant. As a result, there is now a big market for flashmemories.

Each memory cell used in a NAND flash memory has a gate structure on asemiconductor substrate. In this gate structure, a tunnel insulatingfilm, a charge storage film, an interelectrode insulating film, and acontrol electrode are stacked in this order. The gate structure may beof a floating gate type (FG type) having a floating gate electrode madeof polysilicon to be the charge storage film on the tunnel insulatingfilm, a MONOS (Metal-Oxide-Nitride-Oxide-Silicon) type having a chargetrapping film made of silicon nitride to be the charge storage film, ora SONOS (Silicon-Oxide-Nitride-Oxide-Silicon) type having the chargestorage film made of a nitride and the control electrode made ofsilicon.

The threshold voltage of each memory cell can be varied in the followingmanner. By controlling the voltage (control voltage) to be applied tothe control electrode formed on the floating gate or the charge trappingfilm via the interelectrode insulating film, electrons are injected(written) from the substrate into the floating gate electrode or thecharge trapping film via the tunnel insulating film through FN(Fowler-Nordheim) tunneling, or electrons are pulled out of the floatinggate electrode via the tunnel insulating film (erasing in the FG, MONOS,or SONOS type). Also, holes are injected into the charge trapping film,so that the holes and electrons annihilate each other (auxiliary erasingin the MONOS/SONOS type). However, as the tunnel insulating film hasbecome thinner in the trend of miniaturization, attention is now drawnto one serious problem.

To achieve a larger amount of memory, reducing the device size (thememory cell size) is the most effective, but the tunnel insulating filmneeds to be thinner at the same time. SiO₂ film that is widely used asthe tunnel insulating film characteristically experiences more leakagecurrent particularly in a low-field region, called SILC (stress InducedLeakage Current), via the trapping center existing in the SiO₂ film, dueto application of a stress voltage as well as the smaller filmthickness. As a result, the amount of charges passing through the SiO₂film increases, and the data retention properties are degraded. The timeelapsed before the charge amount reaches a destructive value becomesshorter accordingly, or the rewriting performance is degraded. The SILChinders a reduction of the SiO₂ film thickness, and deteriorates thereliability, resulting in great difficulties in miniaturization. Unlessthis trapping center is reduced, a larger amount of memory cannot beexpected by reducing the film thickness of the SiO₂ film.

As described above, as a known cause of degradation of thecharacteristics of the SiO₂ film, an interface layer made of SiO₂ in anamorphous state is formed between the semiconductor substrate and theSiO₂ film, and oxygen defects always exist in the amorphous SiO₂. Thisresults in various traps and leakage sites.

Dangling bond of Si at SiO₂/Si interface is normally terminated withhydrogen. However, the hydrogen is detached from the dangling bond of Siby electrons or holes during a rewriting operation, and does not providea fundamental solution. It has been known that terminating a danglingbond of Si with deuterium is effective, but it remains unclear whetherthe use of deuterium is effective at the SiO₂/Si interface.

As a solution to the problem, the following method has been suggested.Nitrogen is introduced into the SiO₂ film to be the tunnel insulatingfilm, so as to increase the dielectric constant and the physical filmthickness, and reduce the leakage current. However, the effect of thismethod is not sufficient, and the thinnest possible film thickness isnot as thin as expected. This is because the defect formation due to theinsufficient Si—N network is not appropriately restricted.

To counter this problem, there has been a structure in which athree-layer structure having a high-quality silicon nitride filminterposed between silicon oxide films is used as the tunnel insulatingfilm that does not easily allow defect formation, and the siliconnitride film has three-coordinate nitrogen bonds (see JP-A2007-059872(KOKAI)). To form the insulating film having the stackedstructure formed with a SiO₂ layer, a SiN layer, and a SiO₂ layer, thefollowing method is known. After a SiO₂ layer is formed on a Sisubstrate, amorphous Si is deposited and is nitrided to form a nitridelayer made of SiN. The nitride layer is then oxidized, or an oxide layeris deposited by CVD.

However, as a result of a study made by the inventor, the followingproblems were found with the above method. 1) If nitridation isperformed at a high temperature, the amorphous Si is crystallized andagglomerated to form a grain boundary, and the layer thickness of the Silayer fluctuates. 2) The concentration of hydrogen remaining in theamorphous Si might become high. In the case of the problem 1), if theSiO₂ layers are thin, nitrogen penetrates through the lower SiO₂ film,and defects are formed at the interface between the SiO₂ layer and theSi substrate. When a current flows from the Si substrate to the controlelectrode, not only the amount of current in the low electric field orthe medium electric field increases, but also the insulating propertiesare severely degraded due to a local decrease in the layer thickness ofthe nitride layer in a worst case scenario. In the case of the problem2), the reliability deteriorates.

SUMMARY OF THE INVENTION

The present invention has been made in view of these circumstances, andan object thereof is to provide a semiconductor device that has ahigh-quality insulating film in which defects are not easily formed, andexperiences less leakage current. Another object is to provide a methodfor manufacturing such a semiconductor device.

A method for manufacturing a semiconductor device according to a firstaspect of the present invention, includes: forming an amorphous siliconlayer on an insulating layer; introducing oxygen into the amorphoussilicon layer; and forming a silicon oxynitride layer by nitriding theamorphous silicon layer having oxygen introduced thereinto.

A semiconductor device according to a second aspect of the presentinvention, includes an insulating film, the insulating film including: afirst silicon oxide layer formed on a silicon substrate; a siliconoxynitride layer formed on the first silicon oxide layer and havingaverage oxygen concentration in the range of 10 atomic % to 30 atomic %;and a second silicon oxide layer formed on the silicon oxynitride layer.

A semiconductor device according to a third aspect of the presentinvention, includes an insulating film, the insulating film including: afirst silicon oxynitride layer formed on a silicon substrate and havingaverage nitrogen concentration higher than 0 atomic % but not higherthan 10 atomic %; a second silicon oxynitride layer formed on the firstsilicon oxynitride layer and having average oxygen concentration in therange of 10 atomic % to 30 atomic %; and a silicon oxide layer formed onthe second silicon oxynitride layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) to 4(b) are cross-sectional views illustrating theprocedures for manufacturing a semiconductor device according to a firstembodiment;

FIG. 5 shows the variations in the surface roughness after the amorphoussilicon layer is nitrided in the first embodiment with respect to a casewhere oxygen is added and a case where oxygen is not added;

FIG. 6 shows the results of a comparison made on the SILC properties ofthe stacked structure insulating film formed with a SiO₂ layer, a SiONlayer, and a SiO₂ layer in a case where oxygen is added and a case whereoxygen is not added;

FIG. 7 shows the relationship between the conditions for adding oxygento the amorphous silicon layer and the C-V characteristics;

FIG. 8 shows the relationship between the conditions for adding oxygento the amorphous silicon layer and the leakage current;

FIG. 9 shows the relationship between the conditions for adding oxygento the amorphous silicon layer and the C-V characteristics;

FIG. 10 shows the relationship between the conditions for adding oxygento the amorphous silicon layer and the leakage current;

FIG. 11 shows the variation in leakage current observed where theapplied voltage was 4 MV/cm;

FIGS. 12( a) to 15(b) are cross-sectional views illustrating theprocedures for manufacturing a semiconductor device according to asecond embodiment;

FIGS. 16( a) and 16(b) show the differences in erase characteristicsbetween a case where surface nitridation is performed and a case wheresurface nitridation is not performed;

FIG. 17 shows the differences in charge retention properties between acase where surface nitridation is performed and a case where surfacenitridation is not performed;

FIGS. 18( a) to 21(b) are cross-sectional views illustrating theprocedures for manufacturing a semiconductor device according to a thirdembodiment;

FIG. 22 shows the differences in reliability between a case where a heattreatment is carried out in an oxidizing atmosphere after nitridationand a case where the heat treatment is not carried out;

FIGS. 23( a) to 26(b) are cross-sectional views illustrating theprocedures for manufacturing a semiconductor device according to afourth embodiment;

FIGS. 27( a) and 27(b) show the relationship between the heat treatmentconditions in an oxidizing atmosphere and the reliability;

FIG. 28 shows the relationship between the oxidation time and theelectric field required to inject 0.01 A/cm² of electrons;

FIGS. 29( a) to 32(b) are cross-sectional views illustrating theprocedures for manufacturing a semiconductor device according to a fifthembodiment;

FIG. 33 illustrates the effects of adding Ge to the silicon oxide layer;

FIG. 34 is an energy band diagram of a case where Ge is added to thesilicon oxide layer;

FIG. 35 illustrates the reason to define the layer thickness of theamorphous silicon layer;

FIG. 36 illustrates the defined distribution of the nitrogenconcentration in the tunnel insulating film having a three-layer stackedstructure; and

FIG. 37 also illustrates the defined distribution of the nitrogenconcentration in the tunnel insulating film having a three-layer stackedstructure.

DETAILED DESCRIPTION OF THE INVENTION

The following is a description of embodiments of the present invention,with reference to the accompanying drawings.

First Embodiment

Referring to FIGS. 1( a) to 4(b), a method for manufacturing asemiconductor device in accordance with a first embodiment of thepresent invention is described. FIGS. 1( a), 1(c), 1(e), 2(a), 2(c),3(a), 3(c), and 4(a) are cross-sectional views that are takenperpendicularly to the cross-sectional views of FIGS. 1( b), 1(d), 1(f),2(b), 2(d), 3(b), 3(d), and 4(b).

A semiconductor device manufactured by the manufacturing method inaccordance with this embodiment is a FG (floating gate) nonvolatilesemiconductor memory that includes memory cells arranged in a matrixfashion. As shown in FIGS. 4( a) and 4(b), each of the memory cellsincludes: source and drain regions 17 a and 17 b that are formed at adistance from each other on a silicon substrate 1; a tunnel insulatingfilm 5 that is formed on a region 18 of the silicon substrate 1 locatedbetween the source region 17 a and the drain region 17 b to be channel;a floating gate electrode 6 that is formed on the tunnel insulating film5 and serves as a charge storage film having charges stored therein; aninterelectrode insulating film 10 that is formed on the floating gateelectrode 6; and a control gate electrode 11 that is formed on theinterelectrode insulating film 10.

Next, the method for manufacturing the semiconductor device inaccordance with this embodiment is described.

First, as shown in FIGS. 1( a) and 1(b), the silicon substrate 1 dopedwith desired impurities is subjected to diluted hydrofluoric acidtreatment, so as to terminate the surface of the silicon substrate 1with hydrogen. After that, the silicon substrate 1 is left in thechamber of a film forming device (not shown). The chamber is filled onlywith a gas (such as a nitrogen gas) that does not react with or etchsilicon during the manufacturing process. The temperature of the siliconsubstrate 1 is then heated to 700° C., so as to completely detach thehydrogen from the silicon substrate 1.

The chamber is then filled with a N₂ gas having a partial pressure of 30Torr and an O₂ gas having a partial pressure of 3 Torr, for example. Thesurface of the silicon substrate 1 is heated to 1050° C., and ismaintained at that temperature for ten seconds. In this manner, asilicon oxide layer 2 is formed on the silicon substrate 1, as shown inFIGS. 1( c) and 1(d).

An amorphous silicon layer of 2 nm in thickness is then deposited on thesilicon oxide layer 2 with the use of a disilane gas. It is preferablethat the temperature of the silicon substrate 1 at this point is 550° C.or lower. The atmosphere at the time of deposition of the amorphoussilicon layer 3 may contain oxygen, NO, or N₂O. If the atmospherecontains NO or N₂O in this case, the resultant amorphous silicon layercontains a small amount of nitrogen. The temperature of the siliconsubstrate 1 is set at 750° C., and the atmosphere in the chamber ischanged to a N₂ gas having a partial pressure of 30 Torr and an O₂ gashaving a partial pressure of 3 Torr, for example. These conditions aremaintained for ten seconds. In this manner, a small amount of oxygen isadded to the amorphous silicon layer, and an oxygen-added amorphoussilicon layer is formed. After that, the atmosphere in the chamber ischanged to a N₂ gas having a partial pressure of 30 Torr and a NH₃ gashaving a partial pressure of 0.03 Torr, for example. The surface of thesilicon substrate 1 is then set at 750° C., and is maintained at thattemperature for 400 seconds. In this manner, the amorphous silicon layeris nitrided to form a silicon oxynitride layer 3 (see FIGS. 1( e) and1(f)). By the manufacturing method in accordance with this embodiment, anitrogen gas NH₃ diluted with a N₂ gas is used to form the siliconoxynitride layer 3. By diluting the nitrogen gas NH₃ with a dilution gasN₂, a high-quality silicon oxynitride layer having few defects can beformed. This nitriding technique using NH₃ was invented by the inventorof the present invention, and a patent application covering thistechnique was filed (see JP-A 2007-123825(KOKAI)).

The temperature of the silicon substrate 1 is then set at 750° C. orhigher, and a 5-nm thick silicon oxide layer 4 is deposited on thesilicon oxynitride layer 3 by the HTO (High Temperature Oxidation)technique. By doing so, the tunnel insulating film 5 including thesilicon oxide layer 2, the silicon oxynitride layer 3, and the siliconoxide layer 4 is formed as shown in FIGS. 1( e) and 1(f). It should benoted here that, in the drawings illustrating the manufacturingprocedures carried out after this procedure, the tunnel insulating film5 including the silicon oxide layer 2, the silicon oxynitride layer 3,and the silicon oxide layer 4 is shown as a single-layer tunnelinsulating film 5.

A 60-nm thick, phosphorus-doped polycrystalline silicon film 6 to be thefloating gate electrode, and a mask material 7 for device isolation arethen deposited in this order by the CVD (Chemical Vapor Deposition)technique. After that, etching is performed on the mask material 7, thepolycrystalline silicon film 6, and the tunnel insulating film 5 in thisorder by the RIE (Reactive Ion Etching) technique with the use of aresist mask (not shown). Etching is further performed on the exposedregions of the silicon substrate 1, so as to form device isolationgrooves 8 of 100 nm in depth (see FIGS. 2( a) and 2(b)).

A silicon oxide film 9 for device isolation is then deposited on theentire surface, so as to completely fill the device isolation grooves 8.After that, the portion of the silicon oxide film 9 on the surface isremoved by the CMP (Chemical Mechanical Polishing) technique, so thatthe surface is flattened. At this point, the mask material 7 is exposed(see FIGS. 2( c) and 2(d)).

The exposed mask material 7 is selectively removed by etching, and theexposed surface portions of the silicon oxide film 9 are removed byetching with the use of a diluted hydrofluoric acid solution. In thismanner, side faces 6 a of the polycrystalline silicon film 6 arepartially exposed. After that, a 15-nm thick alumina layer 10 a isformed on the entire surface by the ALD (Atomic Layer Deposition)technique. Due to the oxidizing agent used in the film formation by ALD,a very thin silicon oxide layer 10 b is formed at the interface betweenthe alumina layer 10 a and the polycrystalline silicon film 6. As aresult, a 16-nm thick interelectrode insulating film 10 having atwo-layer structure consisting of the alumina layer 10 a and the siliconoxide layer 10 b is formed (see FIGS. 3( a) and 3(b)). In FIGS. 3( a)and 3(b), the interelectrode insulating film 10 having the two-layerstructure of the silicon oxide layer 10 b and the alumina layer 10 a isshown as a single-layer interelectrode insulating film 10, instead of atwo-layer stacked structure.

A polycrystalline silicon layer is then formed on the interelectrodeinsulating film 10 by CVD. After that, a tungsten layer is formed onthis polycrystalline silicon layer, and a heat treatment is carried outto turn the tungsten layer into a tungsten silicide layer. In thismanner, a 100-nm thick conductive film 11 having a two-layer structureconsisting of the polycrystalline silicon layer and the tungstensilicide layer is formed (see FIGS. 3( c) and 3(d)). This conductivefilm 11 serves as the control gate electrode 11. A mask material 12 forRIE is then deposited on the entire surface by CVD. Etching is performedby RIE on the mask material 12, the conductive film 11, theinterelectrode insulating film 10, the polycrystalline silicon film 6,and the tunnel insulating film 5 in this order, so as to form the gatehaving a stacked structure. In this manner, the shape of the gate havingthe stacked structure is defined, and grooves 14 extending in theword-line direction (the direction in which the control gate electrode11 extends) are formed on the side of this gate having the stackedstructure (see FIGS. 3( c) and 3(d)).

As shown in FIGS. 4( a) and 4(b), a silicon oxide film 16 is then formedon the side faces and the upper face of the gate having the stackedstructure by a thermal oxidation technique. After that, the source anddrain regions 17 a and 17 b are formed by an ion injection technique.The region of the silicon substrate 1 located between the source region17 a and the drain region 17 b is the channel region 18. Further, aninterlayer insulating film 19 is formed to cover the entire surface byCVD. After that, a wiring layer and the likes are formed by knowntechniques, so as to complete the nonvolatile semiconductor memory (seeFIGS. 4( a) and 4(b)).

The silicon oxynitride layer in the tunnel insulating film 5 formed byperforming nitridation after oxygen is added to the amorphous siliconlayer has higher flatness and fewer hydrogen radicals. Thus, improvementof not only the charge retention properties but also the reliability canbe expected. For example, FIG. 5 shows the variation in the surfaceroughness under different conditions for forming the silicon oxynitridelayer. FIG. 6 shows the variation in the SILC (Stress Induced LeakageCurrent) properties. More specifically, FIG. 5 shows the surfaceroughness observed where a silicon nitride layer is formed by performingnitridation directly on an amorphous silicon layer, and the surfaceroughness observed where a silicon oxynitride layer is formed byperforming nitridation after oxygen is added to an amorphous siliconlayer. As can be seen from FIG. 5, the surface roughness is reduced byperforming the nitridation after oxygen is added to the amorphoussilicon. This indicates that agglomeration of the amorphous siliconlayer at the time of nitridation is prevented by adding oxygen to theamorphous silicon layer, and the flatness of the silicon oxynitridelayer is increased accordingly.

FIG. 6 shows the results of comparisons of SILC properties between a MOStransistor having a gate insulating film of a stacked structure formedwith SiO₂, SiON, and SiO₂, and a MOS transistor having a gate insulatingfilm of a stacked structure formed with SiO₂, SiN, and SiO₂. As can beseen from FIG. 6, the SILC is reduced by performing the nitridationafter oxygen is added to the amorphous silicon layer. This indicatesthat, as the flatness and the insulation quality are improved, thehydrogen radicals in the amorphous silicon layer are substituted withthe oxygen, and the amount of hydrogen radial becomes smaller. In otherwords, the reliability is improved by performing the nitridation afteroxygen is added to the amorphous silicon layer. In FIG. 6, the ordinateaxis indicates the leakage current J_(g), and the abscissa axisindicates the value obtained by dividing the difference between the gatevoltage V_(g) and the flat-band voltage V_(fb) by the electricallyeffective film thickness T_(eff) of the transistor. This value indicatedby the abscissa axis represents the electric field induced in theinsulating film. This arrangement is made to eliminate the influence ofthe fixed charges in the insulating film, and compare the insulatingproperties at the intensity of the electric field induced in theinsulating film. This is because, since the flat-band voltage V_(fb)shifts in accordance with the amount of fixed charges in the film, theelectric field induced in the insulating film is wrongly estimated whena comparison is made only in terms of the gate voltage V_(g). Theelectrically effective film thickness T_(eff) of the transistor includesnot only the electrically effective film thickness of the gateinsulating film but also the thickness of the depletion layer in thepolysilicon electrode and the thickness of the inversion layer on theside of the semiconductor substrate. When a voltage is applied to a MOSstructure, the voltage is applied not only to the gate insulating filmbut also to the gate electrode and the semiconductor substrate. With thevoltage, an inversion layer is formed on the semiconductor substrateside, and a depletion layer is formed on the gate electrode side. Theselayers are connected as capacitances in series to the capacitance of thegate insulating film. Those series-connected capacitances function asthe electrically effective film thickness when the transistor is on.

When oxygen is added to the amorphous silicon layer, it is necessary topay attention to the temperature, the oxygen partial pressure, the time,the heating-up time, and the timing for introducing oxygen. It ispreferable that the temperature is 700° C. or higher, at which hydrogenis detached from the amorphous silicon layer. However, the temperaturecannot be too high, and should preferably be 800° C. or lower.Furthermore, it is preferable to supply oxygen while the temperature isrising. This is because movement of the silicon is limited by the oxygenentering the amorphous silicon layer and being coupled with the silicon.This is also because, when the amorphous silicon layer is heated in avacuum containing no oxygen at a high temperature, agglomeration isformed as the weak bonds in the amorphous silicon layer turn into stablebonds and crystals that lead to an increase in the roughness.

The oxygen partial pressure should preferably be low so as to reduce theoxidation speed and increase the controllability. More preferably, theoxygen partial pressure should be 50 Torr or lower. However, the oxygenpartial pressure cannot be too low. If it is too low, an activeoxidation region is formed where silicon etching is caused by the oxygenwith the low partial pressure. The oxygen partial pressure shouldpreferably be 10⁻⁴ Torr or higher, which is a passive oxidation regionwhere oxidation is caused at 700° C. to 800° C.

The time required for the temperature rise to a point between 700° C.and 800° C. should preferably be 10 seconds or less. If the temperaturerise takes long while oxygen is being supplied, the 2-nm thick amorphoussilicon layer is completely oxidized before the temperature reaches thetarget temperature, and a SiO₂ layer in which nitrogen is easilydiffused is readily formed. Because of this, a large amount of nitrogenpenetrates through the amorphous silicon oxide layer and the SiO₂ layerbelow the amorphous silicon oxide layer due to the nitridation causedafter that. The Si substrate is then nitrided, and many interface statesare formed. Ideally, while oxygen is being supplied, it is preferablethat the time required for the temperature to reach a point between 700°C. and 800° C. is ten seconds or less, and the oxidation time at atemperature between 700° C. and 800° C. is ten seconds or less. Withinthis time limit, the 2-nm thick amorphous silicon layer is notcompletely oxidized, and nitridation can be performed on the amorphoussilicon layer while nitrogen is restrained from penetrating through theamorphous silicon layer.

Further, to prevent complete oxidation of the amorphous silicon layer,and to oxidize the interface at a low temperature so as not to increasethe interface state density, oxygen should not be supplied while thetemperature is dropping.

Referring now to FIGS. 7 to 11, the relationship between the conditionsfor adding oxygen to the amorphous silicon layer deposited on the SiO₂layer and the electrical characteristics is described. The siliconnitride layer of the transistor having the stack gate structureconsisting of a Si layer, SiO₂ layer, the silicon nitride layer, and anAu layer each having substantially the same capacitance C was formed byseveral different methods, and the electrical characteristics wereexamined. More specifically, the electrical characteristics wereexamined with respect to the case where the silicon nitride layer wasformed by performing the nitridation after oxygen was added to theamorphous silicon layer at the oxidation temperature of 750° C. and inthe oxidation time of ten seconds (in this case, the silicon nitridelayer was a silicon oxynitride layer (SiON layer)) as in thisembodiment, the case as Comparative Example 1 where the silicon nitridelayer was formed by performing the nitridation without oxygen added tothe amorphous silicon layer, and the case as Comparative Example 2 wherethe silicon nitride layer was formed in the same manner as in thisembodiment, except that the oxidation temperature was as high as 850° C.

FIG. 7 shows the results of examinations conducted on the dependence ofthe capacitance C on the gate voltage V_(g), or the C-V characteristics.FIG. 8 shows the results of examinations conducted on the dependence ofthe leakage current J_(g) on the intensity of the electric field inducedin the insulating film (=(V_(g)−V_(fb))/T_(eff)). As can be seen fromthese graphs, in Comparative Example 1, or in a case where the oxidationtime is ten seconds as in this embodiment but the oxidation temperatureis as high as 850° C. as in Comparative Example 2, hysteresis occurs inthe C-V characteristics, and the interface state density and the leakagecurrent are higher than in this embodiment. These facts indicate that,since oxygen was not added to the amorphous silicon layer in ComparativeExample 1, the amorphous silicon layer was agglomerated at the time ofthe nitridation at 700° C., irregularity was caused in the layerthickness, the nitridation became uneven to leave non-nitrided portions(to be more specific, Si dangling bonds), and the nitrogen penetratedthrough the SiO₂ layer below the amorphous silicon layer. The abovefacts also indicate that, since the oxidation temperature was high inComparative example 2, the amorphous silicon layer was agglomerated,irregularity was caused in the layer thickness, the nitridation becameuneven to leave non-nitrided portions, and the nitrogen penetrationoccurred.

Next, electrical characteristics were also examined with respect to acase as Comparative Example 3 where the oxidation temperature and theoxidation time were the same as those in this embodiment (750° C., tenseconds) but oxygen was not supplied while oxygen was added, and a caseas Comparative Example 4 where the oxygen temperature was the same asthat in this embodiment but the oxidation time was 15 seconds. FIG. 9shows the results of examinations conducted on the dependence of thecapacitance C on the gate voltage V_(g), or the C-V characteristics.FIG. 10 shows the results of examinations conducted on the dependence ofthe leakage current J_(g) on the intensity of the electric field inducedin the insulating film (=(V_(g)−V_(fb))/T_(eff)).

As can be seen from these graphs, in both Comparative Examples 3 and 4,large hysteresis occurred in the C-V characteristics, and the interfacestate density and the leakage current became higher than in thisembodiment. These facts indicate that, since there was no oxygen whilethe temperature was rising in Comparative Example 3, the amorphoussilicon layer was agglomerated, irregularity was caused in the layerthickness, the nitridation became uneven to leave non-nitrided portions,and the nitrogen penetration occurred. These facts also indicate that,since the oxidation time was long in Comparative example 4, theamorphous silicon layer was completely oxidized to form a SiO₂ layer,the nitrogen penetrated through the SiO₂ layer, and the Si substrate wasnitrided.

FIG. 11 shows the variation in leakage current observed where theapplied voltage was 4 MV/cm, and the oxidation conditions were varied.FIG. 11 shows the ratio between the leakage current value observed wherethe nitridation was performed without the oxygen addition, and theleakage current value observed where the nitridation was performed afteroxygen was added. For example, the ratio of 50% indicates that theleakage current was reduced by half where oxygen was added. As isapparent from the results shown in FIG. 11, the leakage current can bevery effectively reduced by adding oxygen at a temperature between 700°C. to 800° C. in ten seconds or less. The reasons for this are asdescribed above.

In this manner, a high-quality tunnel insulating film having a stackedstructure formed with a SiO₂ layer, a SiON layer, and a SiO₂ layer canbe realized by adding oxygen to the amorphous silicon layer underappropriate conditions.

The silicon oxynitride layer 3 has a layer thickness of 1.5 nm to 2.5nm, and nitrogen concentration of 20 atomic % to 47 atomic %. In otherwords, 10 atomic % to 30 atomic % of oxygen is contained in the siliconoxynitride layer 3, and at least one of the second nearest neighboratoms of the silicon is nitrogen. This is because oxygen is added to theamorphous silicon layer. In the silicon oxide layer 2, a small amount ofnitrogen is diffused when the amorphous silicon layer is nitrided.Therefore, an average of 10 atomic % of nitrogen is contained at amaximum.

Although an O₂ gas is used when oxygen is added to the amorphous siliconlayer in this embodiment, the gas used at the time of adding oxygen isnot limited to an O₂ gas, and it is possible to use any oxidizing gassuch as NO, N₂O, O₃, O radicals, or O plasma. However, those gasesdiffer from O₂ in oxidation power. Therefore, when one of those gases isused, it is necessary to adjust the oxygen adding conditions in such amanner that the oxygen concentration in the amorphous silicon oxynitridelayer 3 is in the range of 10 atomic % to 30 atomic %.

Although a NH₃ gas is used when the amorphous silicon layer is nitridedin this embodiment, the gas used at the time of nitridation is notlimited to a NH₃ gas, and it is possible to use any nitriding gas suchas NO, N radicals, NH radicals, N₂ radicals, N plasma, NH plasma, or N₂plasma.

Although HTO is used in the process for forming the SiO₂ layer 4 in thisembodiment, some other technique such as CVD or ALD may be used, as longas the SiO₂ layer 4 can be properly deposited.

As described so far, this embodiment can provide a semiconductor devicethat has a high-quality tunnel insulating film in which defects are noteasily formed. This semiconductor device experiences less leakagecurrent.

Second Embodiment

Referring to FIGS. 12( a) to 15(b), a method for manufacturing asemiconductor device in accordance with a second embodiment of thepresent invention is described. FIGS. 12( a), 12(c), 12(e), 13(a),13(c), 14(a), 14(c), and 15(a) are cross-sectional views that are takenperpendicularly to the cross-sectional views of FIGS. 12( b), 12(d),12(f), 13(b), 13(d), 14(b), 14(d), and 15(b).

A semiconductor device manufactured by the manufacturing method inaccordance with this embodiment is a MONOS(Metal-Oxide-Nitride-Oxide-Si) nonvolatile semiconductor memory thatincludes memory cells arranged in a matrix fashion. As shown in FIGS.15( a) and 15(b), each of the memory cells includes: source and drainregions 37 a and 37 b that are formed at a distance from each other on asilicon substrate 1; a tunnel insulating film 25 that is formed on aregion 38 of the silicon substrate 1 located between the source region37 a and the drain region 37 b to be channel; a charge storage film 26that is formed on the tunnel insulating film 25 and is made of aninsulating material in which charges can be stored; an interelectrodeinsulating film 30 that is formed on the charge storage film 26; and acontrol gate electrode 31 that is formed on the interelectrodeinsulating film 30.

Next, the method for manufacturing the semiconductor device inaccordance with this embodiment is described. The manufacturing methodin accordance with this embodiment differs from the manufacturing methodin accordance with the first embodiment, in further including theprocedure for nitriding the silicon oxide layer serving as the baselayer of the amorphous silicon layer before the amorphous silicon layeris formed.

First, as shown in FIGS. 12( a) and 12(b), the silicon substrate 1 dopedwith desired impurities is subjected to diluted hydrofluoric acidtreatment, so as to terminate the surface of the silicon substrate 1with hydrogen. After that, the silicon substrate 1 is placed in thechamber of a film forming device (not shown). The chamber is filled onlywith a gas (such as a nitrogen gas) that does not react with or etchsilicon during the manufacturing process. The temperature of the siliconsubstrate 1 is then heated to 700° C., so as to completely detach thehydrogen from the silicon substrate 1.

The chamber is then filled with a N₂ gas having a partial pressure of 30Torr and an O₂ gas having a partial pressure of 3 Torr, for example. Thesurface of the silicon substrate 1 is heated to 1050° C., and ismaintained at that temperature for ten seconds. In this manner, asilicon oxide layer 22 a is formed on the silicon substrate 1, as shownin FIGS. 12( c) and 12(d). The procedures described so far are the sameprocedures carried out in the first embodiment. Radical nitridation orplasma nitridation is then performed on the surface of the silicon oxidelayer 22 a. By doing so, a nitrogen-containing layer 22 b is formed onthe surface of the silicon oxide layer 22 a, and nitrogen is introducedinto the silicon oxide layer 22 a so that the peak concentration in theprofile becomes 10 atomic % or lower (see FIGS. 12( c) and 12(d)). Inthis manner, a nitrogen-added silicon oxide layer 22 having a two-layerstructure consisting of the silicon oxide layer 22 a and thenitrogen-containing layer 22 b is formed. In this nitriding procedure,this embodiment differs from the first embodiment. Through thisnitriding procedure, the hole injecting efficiency in each memory cellis increased, as will be described later. It should be noted here that,in the drawings illustrating the manufacturing procedures carried outafter this nitriding procedure, the silicon oxide layer 22 having thetwo-layer structure consisting of the silicon oxide layer 22 a and thenitrogen-containing layer 22 b is shown as a single-layer silicon oxidelayer 22.

An amorphous silicon layer of 2 nm in thickness is then deposited on thesilicon oxide layer 22 with the use of a disilane gas. It is preferablethat the temperature of the silicon substrate 1 at this point is 550° C.or lower. The atmosphere at the time of deposition of the amorphoussilicon layer may contain oxygen, NO, or N₂O. If the atmosphere containsNO or N₂O in this case, the resultant amorphous silicon layer contains asmall amount of nitrogen. The temperature of the silicon substrate 1 isset at 750° C., and the atmosphere in the chamber is changed to a N₂ gashaving a partial pressure of 30 Torr and an O₂ gas having a partialpressure of 3 Torr, for example. These conditions are maintained for tenseconds. In this manner, a small amount of oxygen is added to theamorphous silicon layer, and an oxygen-added amorphous silicon layer isformed. As in the first embodiment, the oxidizing temperature at thispoint should preferably be 700° C. or higher, at which the hydrogen inthe amorphous silicon layer is detached. However, the oxidizingtemperature cannot be too high, and should preferably be 800° C. orlower. Furthermore, it is preferable to supply oxygen while thetemperature is rising. Also, as in the first embodiment, the timerequired for the temperature to rise to a point between 700° C. and 800°C. should preferably be ten seconds or less. Ideally, while oxygen isbeing supplied, the heating-up time is ten seconds or less, and theoxidation time is ten seconds or less. After that, the atmosphere in thechamber is changed to a N₂ gas having a partial pressure of 30 Torr anda NH₃ gas having a partial pressure of 0.03 Torr, for example. Thesurface of the silicon substrate 1 is then set at 750° C., and ismaintained at that temperature for 400 seconds. In this manner, theamorphous silicon layer is nitrided to form a silicon oxynitride layer23 (see FIGS. 12( e) and 12(f)). As described in the first embodiment,the silicon oxynitride layer 23 formed by the manufacturing method inaccordance with this embodiment is a high-quality silicon oxynitridelayer having few defects.

The temperature of the silicon substrate 1 is then set at 750° C. orhigher, and a 2.5-nm thick silicon oxide layer 24 is deposited on thesilicon oxynitride layer 23 by HTO. By doing so, the tunnel insulatingfilm 25 including the silicon oxide layer 22, the silicon oxynitridelayer 23, and the silicon oxide layer 24 is formed as shown in FIGS. 12(e) and 12(f). It should be noted here that, in the drawings illustratingthe manufacturing procedures carried out after this procedure, thetunnel insulating film 25 having the three-layer structure consisting ofthe silicon oxide layer 22, the silicon oxynitride layer 23, and thesilicon oxide layer 24 is shown as a single-layer tunnel insulating film25.

A 6-nm thick nitride film 26 to be the charge storage film such as aSi₃N₄ film is then deposited on the tunnel insulating film 25 by CVD,and a mask material 27 for device isolation is deposited by CVD. Afterthat, etching is performed on the mask material 27, the nitride film 26,and the tunnel insulating film 25 in this order by RIE with the use of aresist mask (not shown). Etching is further performed on the exposedregions of the silicon substrate 1, so as to form device isolationgrooves 28 of 100 nm in depth (see FIGS. 13( a) and 13(b)).

A silicon oxide film 29 for device isolation is then deposited on theentire surface, so as to completely fill the device isolation grooves28. After that, the portion of the silicon oxide film 29 on the surfaceis removed by CMP, so that the surface is flattened. At this point, themask material 27 is exposed (see FIGS. 13( c) and 13(d)).

The exposed mask material 27 is selectively removed by etching, and theexposed surface portions of the silicon oxide film 29 are removed byetching with the use of a diluted hydrofluoric acid solution. Afterthat, a 15-nm thick alumina layer 30 a to be the interelectrodeinsulating film is formed on the entire surface by ALD. Due to theoxidizing agent used in the film formation by ALD, a very thin siliconoxide layer 30 b is formed at the interface between the alumina layer 30a and the nitride film 26. As a result, a 16-nm thick interelectrodeinsulating film 30 having a two-layer structure consisting of thealumina layer 30 a and the silicon oxide layer 30 b is formed (see FIGS.14( a) and 14(b)). It should be noted here that, in the drawingsillustrating the manufacturing procedures carried out after thisprocedure, the interelectrode insulating film 30 having the two-layerstructure of the silicon oxide layer 30 b and the alumina layer 30 a isshown as a single-layer interelectrode insulating film 30.

A 100-nm thick conductive film 31 having a two-layer structureconsisting of a polycrystalline silicon layer and a tungsten silicidelayer is formed to be the control gate electrode on the interelectrodeinsulating film 30 in the same manner as in the first embodiment.Further, a mask material 32 for RIE is deposited by CVD. After that,etching is performed by RIE with the use of a resist mask (not shown) onthe mask material 32, the conductive film 31, the interelectrodeinsulating film 30, the charge storage film 26, and the tunnelinsulating film 25 in this order, so as to form the gate having astacked structure. In this manner, the shape of the gate having thestacked structure is defined, and grooves 34 extending in the word-linedirection (the direction in which the control gate electrode 31 extends)are formed on the side of this gate having the stacked structure (seeFIGS. 14( c) and 14(d)).

As shown in FIGS. 15( a) and 15(b), a silicon oxide film 36 is thenformed on the side faces and the upper face of the gate having thestacked structure by a thermal oxidation technique. After that, thesource and drain regions 37 a and 37 b are formed by an ion injectiontechnique. The region of the silicon substrate 1 located between thesource region 37 a and the drain region 37 b is the channel region 38.Further, an interlayer insulating film 39 is formed to cover the entiresurface by CVD. After that, a wiring layer and the likes are formed byknown techniques, so as to complete the nonvolatile semiconductor memory(see FIGS. 15( a) and 15(b)).

By performing nitridation on the surface of the SiO₂ layer before thedeposition of the amorphous silicon layer in the above described manner,the charge retention properties and the reliability described in thefirst embodiment can be further improved, and an increase in erasingefficiency can be expected. FIGS. 16( a) and 16(b) show the differencesin the charge retention properties between a case with the surfacenitridation and a case without the surface nitridation. FIG. 17 showsthe differences in the erase characteristics. In FIGS. 16( a) and 16(b),the abscissa axis indicates the charge retention time, and the ordinateaxis indicates the difference between the flat-band voltage V_(fb) andthe initial flat-band voltage V_(fbini). In FIG. 17, the abscissa axisindicates the initial erase field, and the ordinate axis indicates theflat-band voltage. As can be seen from FIGS. 16( a), 16(b), and 17, byperforming nitridation on the surface of the SiO₂ layer before theamorphous silicon layer is deposited, the retention properties and theerase characteristics are both improved. The retention properties areimproved, because dangling bonds are formed on the surface of the SiO₂layer, and the flatness of the amorphous silicon layer formed on theSiO₂ layer is increased. Accordingly, the insulating properties arefurther improved. Also, diffusion of nitrogen into the SiO₂ layer isprevented by introducing nitrogen into the surface of the SiO₂ layer,and nitriding species are prevented from penetrating through the SiO₂layer and nitriding the Si substrate when the amorphous silicon layer islater nitrided. As a result, formation of interface states to be theorigin of low-field leakage current is restricted. The erasecharacteristics are improved because, as nitrogen is introduced into theSiO₂ layer, the hole barrier of the SiO₂ layer becomes smaller, and thedensity of hole current tunneling from the substrate side to the chargestorage film side becomes higher. As a result, the erasing efficiency ofthe electrons in the charge storage film becomes higher.

In this manner, a high-quality tunnel insulating film having athree-layer stacked structure formed with a SiO₂ layer, a SiON layer,and a SiO₂ layer can be realized by nitriding the surface of the lowerSiO₂ layer before the amorphous silicon layer is deposited.

The silicon oxynitride layer 23 has a layer thickness of 1.5 nm to 2.5nm, and nitrogen concentration of 20 atomic % to 47 atomic %. In otherwords, 10 atomic % to 30 atomic % of oxygen is contained in the siliconoxynitride layer 23, and at least one of the second nearest neighboratoms of the silicon is nitrogen. This is because oxygen is added to theamorphous silicon layer. In the silicon oxide layer 22, a small amountof nitrogen is diffused when the amorphous silicon layer is nitrided.Therefore, an average of 10 atomic % of nitrogen is contained at amaximum.

In this embodiment, radical nitridation or plasma nitridation isperformed on the surface of the silicon oxide layer 22. However, toprevent nitrogen from penetrating through the silicon oxide layer 22 andnitriding the silicon substrate, it is not necessary to perform radicalnitridation or plasma nitridation, and NH₃ or NO may be used instead.

Although an O₂ gas is used when oxygen is added to the amorphous siliconlayer in this embodiment, the gas used at the time of adding oxygen isnot limited to an O₂ gas, and it is possible to use any oxidizing gassuch as NO, N₂O, O₃, O radicals, or O plasma. However, those gasesdiffer from O₂ in oxidation power. Therefore, when one of those gases isused, it is necessary to adjust the oxygen adding conditions in such amanner that the oxygen concentration in the silicon oxynitride layer 23is in the range of 10 atomic % to 30 atomic %.

Although a NH₃ gas is used when the amorphous silicon layer is nitridedin this embodiment, the gas used at the time of nitridation is notlimited to a NH₃ gas, and it is possible to use any nitriding gas suchas NO, N radicals, NH radicals, N₂ radicals, N plasma, NH plasma, or N₂plasma.

Although HTO is used in the process for forming the SiO₂ layer 24 inthis embodiment, some other technique such as CVD or ALD may be used, aslong as the SiO₂ layer 24 can be properly deposited.

As described so far, this embodiment can provide a semiconductor devicethat has a high-quality tunnel insulating film in which defects are noteasily formed. This semiconductor device experiences less leakagecurrent.

Third Embodiment

Referring now to FIGS. 18( a) to 21(b), a method for manufacturing asemiconductor device in accordance with a third embodiment of thepresent invention is described. FIGS. 18( a), 18(c), 18(e), 19(a),19(c), 20(a), 20(c), and 21(a) are cross-sectional views that are takenperpendicularly to the cross-sectional views of FIGS. 18( b), 18(d),18(f), 19(b), 19(d), 20(b), 20(d), and 21(b).

A semiconductor device in accordance with this embodiment is a MONOSnonvolatile semiconductor memory that includes memory cells arranged ina matrix fashion. As shown in FIGS. 21( a) and 21(b), each of the memorycells includes: source and drain regions 57 a and 57 b that are formedat a distance from each other on a silicon substrate 1; a tunnelinsulating film 45 that is formed on a region 58 of the siliconsubstrate 1 located between the source region 57 a and the drain region57 b; a charge storage film 46 that is formed on the tunnel insulatingfilm 45 and is made of an insulating material in which charges can bestored; an interelectrode insulating film 50 that is formed on thecharge storage film 46; and a control gate electrode 51 that is formedon the interelectrode insulating film 50.

Next, the method for manufacturing the semiconductor device inaccordance with this embodiment is described. The manufacturing methodin accordance with this embodiment differs from the manufacturing methodin accordance with the second embodiment, in further including theprocedure for oxidizing a silicon oxynitride layer before a siliconoxide layer is formed on the silicon oxynitride layer.

First, as shown in FIGS. 18( a) and 18(b), the silicon substrate 1 dopedwith desired impurities is subjected to diluted HF treatment, so as toterminate the surface of the silicon substrate 1 with hydrogen. Afterthat, the silicon substrate 1 is placed in the chamber of a film formingdevice. The chamber is filled only with a gas (such as a nitrogen gas)that does not react with or etch silicon during the manufacturingprocess. The temperature of the silicon substrate 1 is then heated to700° C., so as to completely detach the hydrogen from the siliconsubstrate 1.

The chamber is then filled with a N₂ gas having a partial pressure of 30Torr and an O₂ gas having a partial pressure of 3 Torr, for example. Thesurface of the silicon substrate 1 is heated to 1050° C., and ismaintained at that temperature for ten seconds. In this manner, asilicon oxide layer 42 a is formed on the silicon substrate 1, as shownin FIGS. 18( c) and 18(d). Radical nitridation or plasma nitridation isthen performed on the surface of the silicon oxide layer 42 a. By doingso, a nitrogen-containing layer 42 b is formed on the surface of thesilicon oxide layer 42 a, and nitrogen is introduced into the siliconoxide layer 42 a so that the peak concentration in the profile becomes10 atomic % or lower (see FIGS. 18( c) and 18(d)). In the same manner asin the second embodiment, a nitrogen-added silicon oxide layer 42 havinga two-layer structure consisting of the silicon oxide layer 42 a and thenitrogen-containing layer 42 b is formed. Through this nitridingprocedure, the hole injecting efficiency in each memory cell isincreased. It should be noted here that, in the drawings illustratingthe manufacturing procedures carried out after this nitriding procedure,the silicon oxide layer 42 having the two-layer structure consisting ofthe silicon oxide layer 42 a and the nitrogen-containing layer 42 b isshown as a single-layer silicon oxide layer 42.

An amorphous silicon layer of 2 nm in thickness is then deposited on thesilicon oxide layer 42 with the use of a disilane gas. It is preferablethat the substrate temperature at this point is 550° C. or lower. Theatmosphere at the time of deposition of the amorphous silicon layer maycontain oxygen, NO, or N₂O. If the atmosphere contains NO or N₂O in thiscase, the resultant amorphous silicon layer contains a small amount ofnitrogen. The temperature of the silicon substrate 1 is set at 750° C.,and the atmosphere in the chamber is changed to a N₂ gas having apartial pressure of 30 Torr and an O₂ gas having a partial pressure of 3Torr, for example. These conditions are maintained for ten seconds. Inthis manner, a small amount of oxygen is added to the amorphous siliconlayer, and an oxygen-added amorphous silicon layer is formed. As in thefirst embodiment, the oxidizing temperature at this point shouldpreferably be 700° C. or higher, at which the hydrogen in the amorphoussilicon layer is detached. However, the oxidizing temperature cannot betoo high, and should preferably be 800° C. or lower. Furthermore, it ispreferable to supply oxygen while the temperature is rising. Also, thetime required for the temperature to rise to a point between 700° C. and800° C. should preferably be ten seconds or less. Ideally, while oxygenis being supplied, the heating-up time is ten seconds or less, and theoxidation time is ten seconds or less. After that, the atmosphere in thechamber is changed to a N₂ gas having a partial pressure of 30 Torr anda NH₃ gas having a partial pressure of 0.03 Torr, for example. Thesurface of the silicon substrate 1 is then set at 750° C., and ismaintained at that temperature for 400 seconds. In this manner, theamorphous silicon layer is nitrided to form a silicon oxynitride layer43 (see FIGS. 18( e) and 18(f)). As described in the first embodiment,the silicon oxynitride layer 43 formed by the manufacturing method inaccordance with this embodiment is a high-quality silicon oxynitridelayer having few defects.

The temperature of the silicon substrate 1 is then set at 800° C., andthe atmosphere in the chamber is changed to a N₂ gas having a partialpressure of 30 Torr and an O₂ gas having a partial pressure of 0.03Torr. These conditions are maintained for 100 seconds. As a result, theSi—N—H in the silicon oxynitride layer 43 is oxidized, and the hydrogenradicals are reduced. Thus, a high-quality silicon oxynitride layer 43having high reliability can be formed. This procedure is not carried outin the first and second embodiments.

The temperature of the silicon substrate 1 is then set at 750° C. orhigher, and a 2.5-nm thick silicon oxide layer 44 is deposited on thesilicon oxynitride layer 43 by HTO. By doing so, the tunnel insulatingfilm 45 including the silicon oxide layer 42, the silicon oxynitridelayer 43, and the silicon oxide layer 44 is formed as shown in FIGS. 18(e) and 18(f). It should be noted here that, in the drawings illustratingthe manufacturing procedures carried out after this procedure, thetunnel insulating film 45 having the three-layer structure consisting ofthe silicon oxide layer 42, the silicon oxynitride layer 43, and thesilicon oxide layer 44 is shown as a single-layer tunnel insulating film45.

A 6-nm thick nitride film to be the charge storage film 46 such as aSi₃N₄ film is then deposited on the tunnel insulating film 45 by CVD,and a mask material 47 for device isolation is deposited by CVD. Afterthat, etching is performed on the mask material 47, the nitride film(the charge storage film) 46, and the tunnel insulating film 45 in thisorder by RIE with the use of a resist mask (not shown). Etching isfurther performed on the exposed regions of the silicon substrate 1, soas to form device isolation grooves 48 of 100 nm in depth, as shown inFIGS. 19( a) and 19(b).

A silicon oxide film 49 for device isolation is then deposited on theentire surface, so as to completely fill the device isolation grooves48. After that, the portion of the silicon oxide film 49 on the surfaceis removed by CMP, so that the surface is flattened. At this point, themask material 47 is exposed (see FIGS. 19( c) and 19(d)).

The exposed mask material 47 is selectively removed by etching, and theexposed surface portions of the silicon oxide film 49 are removed byetching with the use of a diluted hydrofluoric acid solution. Afterthat, a 15-nm thick alumina layer 50 a to be the interelectrodeinsulating film is formed on the entire surface by ALD. Due to theoxidizing agent used in the film formation by ALD, a very thin siliconoxide layer 50 b is formed at the interface between the alumina layer 50a and the nitride film (the charge storage film) 46. As a result, a16-nm thick interelectrode insulating film 50 having a two-layerstructure consisting of the alumina layer 50 a and the silicon oxidelayer 50 b is formed (see FIGS. 20( a) and 20(b)).

A 100-nm thick conductive film 51 having a two-layer structureconsisting of a polycrystalline silicon layer and a tungsten silicidelayer is formed to be the control gate electrode on the interelectrodeinsulating film 50 in the same manner as in the first embodiment.Further, a mask material 52 for RIE is deposited by CVD. After that,etching is performed by RIE with the use of a resist mask (not shown) onthe mask material 52, the conductive film 51, the interelectrodeinsulating film 50, the charge storage film 56, and the tunnelinsulating film 55 in this order, so as to form the gate having astacked structure. In this manner, the shape of the gate having thestacked structure is defined, and grooves 54 extending in the word-linedirection (the direction in which the control gate electrode 51 extends)are formed on the side of this gate having the stacked structure (seeFIGS. 20( c) and 20(d)).

As shown in FIGS. 21( a) and 21(b), a silicon oxide film 56 is thenformed on the side faces and the upper face of the gate having thestacked structure by a thermal oxidation technique. After that, thesource and drain regions 57 a and 57 b are formed by an ion injectiontechnique. The region of the silicon substrate 1 located between thesource region 57 a and the drain region 57 b is the channel region 58.Further, an interlayer insulating film 59 is formed to cover the entiresurface by CVD. After that, a wiring layer and the likes are formed byknown techniques, so as to complete the nonvolatile semiconductor memory(see FIGS. 21( a) and 21(b)).

By carrying out a heat treatment in an oxidizing atmosphere after theamorphous silicon layer is nitrided in the above described manner, thereliability described in the first embodiment can be further increased.FIG. 22 shows the differences in SILC properties or reliability betweena case where a heat treatment is carried out in an oxidizing atmosphereafter the nitridation, and a case where the heat treatment is notcarried out. As can be seen from FIG. 22, the SILC is reduced bycarrying out the heat treatment in an oxidizing atmosphere after theamorphous silicon layer is nitrided. This is because Si—N—H in thesilicon oxynitride layer 43 is oxidized, and the hydrogen radicals arereduced.

In this manner, a high-quality stacked structure formed with a SiO₂layer, a SiON layer, and a SiO₂ layer can be realized by carrying out aheat treatment in an oxidizing atmosphere after the amorphous siliconlayer is nitrided.

The silicon oxynitride layer 43 has a layer thickness of 1.5 nm to 2.5nm, and nitrogen concentration of 20 atomic % to 47 atomic %. In otherwords, 10 atomic % to 30 atomic % of oxygen is contained in the siliconoxynitride layer 43, and at least one of the second nearest neighboratoms of the silicon is nitrogen. This is because oxygen is added to theamorphous silicon layer. In the silicon oxide layer 42, a small amountof nitrogen is diffused when the amorphous silicon layer is nitrided.Therefore, an average of 10 atomic % of nitrogen is contained at amaximum.

In this embodiment, radical nitridation or plasma nitridation isperformed on the surface of the silicon oxide layer 42. However, toprevent nitrogen from penetrating through the silicon oxide layer 42 andnitriding the silicon substrate, it is not necessary to perform radicalnitridation or plasma nitridation, and NH₃ or NO may be used instead.

Although an O₂ gas is used when oxygen is added to the amorphous siliconlayer in this embodiment, the gas used at the time of adding oxygen isnot limited to an O₂ gas, and it is possible to use any oxidizing gassuch as NO, N₂O, O₃, O radicals, or O plasma. However, those gasesdiffer from O₂ in oxidation power. Therefore, when one of those gases isused, it is necessary to adjust the oxygen adding conditions in such amanner that the oxygen concentration in the silicon oxynitride layer 43is in the range of 10 atomic % to 30 atomic %.

Although a NH₃ gas is used when the amorphous silicon layer is nitridedin this embodiment, the gas used at the time of nitridation is notlimited to a NH₃ gas, and it is possible to use any nitriding gas suchas NO, N radicals, NH radicals, N₂ radicals, N plasma, NH plasma, or N₂plasma.

Although HTO is used in the process for forming the SiO₂ layer in thisembodiment, some other technique such as CVD or ALD may be used, as longas the SiO₂ layer can be properly deposited.

As described so far, this embodiment can provide a semiconductor devicethat has a high-quality tunnel insulating film in which defects are noteasily formed. This semiconductor device experiences less leakagecurrent.

Fourth Embodiment

Referring now to FIGS. 23( a) to 26(b), a method for manufacturing asemiconductor device in accordance with a fourth embodiment of thepresent invention is described. FIGS. 23( a), 23(c), 23(e), 24(a),24(c), 25(a), 25(c), and 26(a) are cross-sectional views that are takenperpendicularly to the cross-sectional views of FIGS. 23( b), 23(d),23(f), 24(b), 24(d), 25(b), 25(d), and 26(b).

A semiconductor device in accordance with this embodiment is a MONOSnonvolatile semiconductor memory that includes memory cells arranged ina matrix fashion. As shown in FIGS. 26( a) and 26(b), each of the memorycells includes: source and drain regions 77 a and 77 b that are formedat a distance from each other on a silicon substrate 1; a tunnelinsulating film 65 that is formed on a region 78 of the siliconsubstrate 1 located between the source region 77 a and the drain region77 b; a charge storage film 66 that is formed on the tunnel insulatingfilm 65 and is made of an insulating material in which charges can bestored; an interelectrode insulating film 70 that is formed on thecharge storage film 66; and a control gate electrode 71 that is formedon the interelectrode insulating film 70.

Next, the method for manufacturing the semiconductor device inaccordance with this embodiment is described. The manufacturing methodin accordance with this embodiment differs from the manufacturing methodin accordance with the third embodiment, in further including theprocedure for oxidizing the tunnel insulating film including a siliconoxide layer, a silicon oxynitride layer, and a silicon oxide layerimmediately after the formation of the silicon oxide layer on thesilicon oxynitride layer.

First, as shown in FIGS. 23( a) and 23(b), the silicon substrate 1 dopedwith desired impurities is subjected to diluted HF treatment, so as toterminate the surface of the silicon substrate 1 with hydrogen. Afterthat, the silicon substrate 1 is placed in the chamber of a film formingdevice. The chamber is filled only with a gas (such as a nitrogen gas)that does not react with or etch silicon during the manufacturingprocess. The temperature of the silicon substrate 1 is then heated to700° C., so as to completely detach the hydrogen from the siliconsubstrate 1.

The chamber is then filled with a N₂ gas having a partial pressure of 30Torr and an O₂ gas having a partial pressure of 3 Torr, for example. Thesurface of the silicon substrate 1 is heated to 1050° C., and ismaintained at that temperature for ten seconds. In this manner, asilicon oxide layer 62 a is formed on the silicon substrate 1, as shownin FIGS. 23( c) and 23(d). Radical nitridation or plasma nitridation isthen performed on the surface of the silicon oxide layer 62 a. By doingso, a nitrogen-containing layer 62 b is formed on the surface of thesilicon oxide layer 62 a, and nitrogen is introduced into the siliconoxide layer 62 a so that the peak concentration in the profile becomes10 atomic % or lower (see FIGS. 23( c) and 23(d)). In the same manner asin the second embodiment, a nitrogen-added silicon oxide layer 62 havinga two-layer structure consisting of the silicon oxide layer 62 a and thenitrogen-containing layer 62 b is formed. Through this nitridingprocedure, the hole injecting efficiency in each memory cell isincreased. It should be noted here that, in the drawings illustratingthe manufacturing procedures carried out after this nitriding procedure,the silicon oxide layer 62 having the two-layer structure consisting ofthe silicon oxide layer 62 a and the nitrogen-containing layer 62 b isshown as a single-layer silicon oxide layer 62.

An amorphous silicon layer of 2 nm in thickness is then deposited on thesilicon oxide layer 62 with the use of a disilane gas. It is preferablethat the substrate temperature at this point is 550° C. or lower. Theatmosphere at the time of deposition of the amorphous silicon layer maycontain oxygen, NO, or N₂O. If the atmosphere contains NO or N₂O in thiscase, the resultant amorphous silicon layer contains a small amount ofnitrogen.

The temperature of the silicon substrate 1 is then set at 750° C., andthe atmosphere in the chamber is changed to a N₂ gas having a partialpressure of 30 Torr and an O₂ gas having a partial pressure of 3 Torr,for example. These conditions are maintained for ten seconds. In thismanner, a small amount of oxygen is added to the amorphous siliconlayer, and an oxygen-added amorphous silicon layer is formed. As in thefirst embodiment, the oxidizing temperature at this point shouldpreferably be 700° C. or higher, at which the hydrogen in the amorphoussilicon layer is detached. However, the oxidizing temperature cannot betoo high, and should preferably be 800° C. or lower. Furthermore, it ispreferable to supply oxygen while the temperature is rising. Also, as inthe first embodiment, the time required for the temperature to rise to apoint between 700° C. and 800° C. should preferably be ten seconds orless. Ideally, while oxygen is being supplied, the heating-up time isten seconds or less, and the oxidation time is ten seconds or less.After that, the atmosphere in the chamber is changed to a N₂ gas havinga partial pressure of 30 Torr and a NH₃ gas having a partial pressure of0.03 Torr, for example. The surface of the silicon substrate 1 is thenset at 750° C., and is maintained at that temperature for 400 seconds.In this manner, the amorphous silicon layer is nitrided to form asilicon oxynitride layer 63 (see FIGS. 23( e) and 23(f)). As describedin the first embodiment, the silicon oxynitride layer 63 formed by themanufacturing method in accordance with this embodiment is ahigh-quality silicon oxynitride layer having few defects.

The temperature of the silicon substrate 1 is then set at 800° C., andthe atmosphere in the chamber is changed to a N₂ gas having a partialpressure of 30 Torr and an O₂ gas having a partial pressure of 0.03Torr. These conditions are maintained for 100 seconds. As a result, theSi—N—H in the silicon oxynitride layer 63 is oxidized, and the hydrogenradicals are reduced. Thus, a high-quality silicon oxynitride layer 63having high reliability can be formed.

The temperature of the silicon substrate 1 is then set at 750° C. orhigher, and a 2.5-nm thick silicon oxide layer 64 is deposited on thesilicon oxynitride layer 63 by HTO. By doing so, the tunnel insulatingfilm 65 including the silicon oxide layer 62, the silicon oxynitridelayer 63, and the silicon oxide layer 64 is formed as shown in FIGS. 23(e) and 23(f). The temperature of the silicon substrate 1 is then set at950° C., and the atmosphere in the chamber is changed to a N₂ gas havinga partial pressure of 760 Torr and an O₂ gas having a partial pressureof 3 Torr. These conditions are maintained for one hour. By doing so,the small amount of hydrogen radicals in the tunnel insulating film 65is oxidized, and the defects in the film are oxygen-terminated. Thus,the tunnel insulating film 65 can have higher quality and reliability.It should be noted here that, in the drawings illustrating themanufacturing procedures carried out after this procedure, the tunnelinsulating film 65 having the three-layer structure consisting of thesilicon oxide layer 62, the silicon oxynitride layer 63, and the siliconoxide layer 64 is shown as a single-layer tunnel insulating film 65.

A 6-nm thick nitride film 66 to be the charge storage film such as aSi₃N₄ film is then deposited on the tunnel insulating film 65 by CVD,and a mask material 67 for device isolation is deposited by CVD. Afterthat, etching is performed on the mask material 67, the nitride film(the charge storage film) 66, and the tunnel insulating film 65 in thisorder by RIE with the use of a resist mask (not shown). Etching isfurther performed on the exposed regions of the silicon substrate 1, soas to form device isolation grooves 68 of 100 nm in depth, as shown inFIGS. 24( a) and 24(b).

A silicon oxide film 69 for device isolation is then deposited on theentire surface, so as to completely fill the device isolation grooves68. After that, the portion of the silicon oxide film 69 on the surfaceis removed by CMP, so that the surface is flattened. At this point, themask material 67 is exposed (see FIGS. 24( c) and 24(d)).

The exposed mask material 67 is selectively removed by etching, and theexposed surface portions of the silicon oxide film 69 are removed byetching with the use of a diluted hydrofluoric acid solution. Afterthat, a 15-nm thick alumina layer 70 a to be the interelectrodeinsulating film is formed on the entire surface by ALD. Due to theoxidizing agent used in the film formation by ALD, a very thin siliconoxide layer 70 b is formed at the interface between the alumina layer 70a and the nitride film 66. As a result, a 16-nm thick interelectrodeinsulating film 70 having a two-layer structure consisting of thesilicon oxide layer 70 b and the alumina layer 70 a is formed (see FIGS.25( a) and 25(b)).

A 100-nm thick conductive film 71 having a two-layer structureconsisting of a polycrystalline silicon layer and a tungsten silicidelayer is formed to be the control gate electrode on the interelectrodeinsulating film 70 in the same manner as in the first embodiment.Further, a mask material 72 for RIE is deposited by CVD. After that,etching is performed by RIE with the use of a resist mask (not shown) onthe mask material 72, the conductive film 71, the interelectrodeinsulating film 70, the charge storage film 66, and the tunnelinsulating film 65 in this order, so as to form the gate having astacked structure. In this manner, the shape of the gate having thestacked structure is defined, and grooves 73 extending in the word-linedirection (the direction in which the control gate electrode 71 extends)are formed on the side of this gate having the stacked structure (seeFIGS. 25( c) and 25(d)).

As shown in FIGS. 26( a) and 26(b), a silicon oxide film 74 is thenformed on the side faces and the upper face of the gate having thestacked structure by a thermal oxidation technique. After that, thesource and drain regions 77 a and 77 b are formed by an ion injectiontechnique. The region of the silicon substrate 1 located between thesource region 77 a and the drain region 77 b is the channel region 78.Further, an interlayer insulating film 79 is formed to cover the entiresurface by CVD (see FIGS. 26( a) and 26(b)). After that, a wiring layerand the likes are formed by known techniques, so as to complete thenonvolatile semiconductor memory.

By carrying out a heat treatment in an oxidizing atmosphere after theoxide layer is deposited by HTO in the above described manner, thereliability described in the first embodiment and the third embodimentcan be further increased. However, just carrying out a heat treatmentwill not suffice. To achieve desired characteristics, it is essentialthat a heat treatment is carried out in an oxidizing atmosphere undercertain heat treatment conditions.

Referring now to FIGS. 27( a) and 27(b), the relationship between theheat treatment conditions in an oxidizing atmosphere and the reliabilityis described. FIGS. 27( a) and 27(b) show the SILC properties, with theoxygen partial pressure and the oxidation time being varied. Morespecifically, FIG. 27( a) shows the dependence of the electric field onthe oxidation time observed when a leakage current of 10⁻⁷ A/cm² flowsbefore and after the 0.01 C/cm² injection. FIG. 27( b) shows theelectric field at the time of the 0.01 C/cm² injection. The oxidationtemperature is 950° C. As can be seen from FIG. 27( a), no improvementsare observed in a N₂ atmosphere, but the electric field, which is 1MV/cm where the heat treatment is not carried out, is increased toapproximately 5.5 MV/cm by the heat treatment in an oxidizingatmosphere. This indicates that the SILC properties are greatly improvedby performing oxidation at 950° C. for ten minutes or longer, regardlessof the pressure. This result confirms that the heat treatment in anoxidizing atmosphere for ten minutes or longer is essential in improvingthe reliability. As for the oxidation temperature, the same effects asabove can be expected when the oxidation temperature is in the range of900° C. to 950° C.

However, excessive oxidation is not preferable to achieve high injectionefficiency, even though the reliability becomes higher. FIG. 28 showsthe relationship between the oxidation time and the electric fieldrequired to inject 0.01 A/cm² of electrons. As can be seen from FIG. 28,the induced electric field is increased by performing oxidation for onehour or longer. Also, the induced electric field is increased, if theoxygen partial pressure is too high. This is because the oxygenconcentration in the oxide layer is reduced by the oxidation, and theelectron injection efficiency becomes lower. Therefore, oxidationperformed for ten minutes or longer, but not longer than one hour, isessential in increasing the reliability while maintaining high injectionefficiency.

In this manner, a high-quality stacked structure formed with a SiO₂layer, a SiON layer, and a SiO₂ layer can be realized by carrying out aheat treatment in an optimum oxidizing atmosphere after an oxide layeris deposited by HTO.

The silicon oxynitride layer 63 has a layer thickness of 1.5 nm to 2.5nm, and nitrogen concentration of 20 atomic % to 47 atomic %. In otherwords, 10 atomic % to 30 atomic % of oxygen is contained in the siliconoxynitride layer 63, and at least one of the second nearest neighboratoms of the silicon is nitrogen. This is because oxygen is added to theamorphous silicon layer. In the silicon oxide layer 62, a small amountof nitrogen is diffused when the amorphous silicon layer is nitrided.Therefore, an average of 10 atomic % of nitrogen is contained at amaximum.

In this embodiment, radical nitridation or plasma nitridation isperformed on the surface of the silicon oxide layer. However, to preventnitrogen from penetrating through the silicon oxide layer, it is notnecessary to perform radical nitridation or plasma nitridation, and NH₃or NO may be used instead.

Although an O₂ gas is used when oxygen is added to the amorphous siliconlayer in this embodiment, the gas used at the time of adding oxygen isnot limited to an O₂ gas, and it is possible to use any oxidizing gassuch as NO, N₂O, O₃, O radicals, or O plasma. However, those gasesdiffer from O₂ in oxidation power. Therefore, when one of those gases isused, it is necessary to adjust the oxygen adding conditions in such amanner that the oxygen concentration in the silicon oxynitride layer isin the range of 10 atomic % to 30 atomic %.

Although a NH₃ gas is used when the amorphous silicon layer is nitridedin this embodiment, the gas used at the time of nitridation is notlimited to a NH₃ gas, and it is possible to use any nitriding gas suchas NO, N radicals, NH radicals, N₂ radicals, N plasma, NH plasma, or N₂plasma.

Although HTO is used in the process for forming the SiO₂ layer 64 inthis embodiment, some other technique such as CVD or ALD may be used, aslong as the SiO₂ layer 64 can be properly deposited.

In this embodiment, a heat treatment is carried out in an oxidizingatmosphere after the oxide layer 64 is deposited by HTO. This greatlyreduces the hydrogen in the stacked structure formed with a SiO₂ layer,a SiON layer, and a SiO₂ layer. Accordingly, as long as a heat treatmentis carried out in an oxidizing atmosphere after the oxide layer 64 isdeposited by HTO, the temperature at which oxygen is added to theamorphous silicon layer and the temperature at which nitridation isperformed on the amorphous silicon layer may not be very high, and maybe 400° C. or higher.

As described so far, this embodiment can provide a semiconductor devicethat has a high-quality tunnel insulating film in which defects are noteasily formed. This semiconductor device experiences less leakagecurrent.

Fifth Embodiment

Referring now to FIGS. 29( a) to 32(b), a method for manufacturing asemiconductor device in accordance with a fifth embodiment of thepresent invention is described. FIGS. 29( a), 29(c), 29(e), 30(a),30(c), 31(a), 31(c), and 32(a) are cross-sectional views that are takenperpendicularly to the cross-sectional views of FIGS. 29( b), 29(d),29(f), 30(b), 30(d), 31(b), 31(d), and 32(b).

A semiconductor device in accordance with this embodiment is a MONOSnonvolatile semiconductor memory that includes memory cells arranged ina matrix fashion. As shown in FIGS. 32( a) and 32(b), each of the memorycells includes: source and drain regions 97 a and 97 b that are formedat a distance from each other on a silicon substrate 1; a tunnelinsulating film 85 that is formed on a region 98 of the siliconsubstrate 1 located between the source region 97 a and the drain region97 b to be channel; a charge storage film 86 that is formed on thetunnel insulating film 85 and is made of an insulating material in whichcharges can be stored; an interelectrode insulating film 90 that isformed on the charge storage film 86; and a control gate electrode 91that is formed on the interelectrode insulating film 90.

Next, the method for manufacturing the semiconductor device inaccordance with this embodiment is described. The manufacturing methodin accordance with this embodiment differs from the manufacturing methodin accordance with the fourth embodiment, in further including theprocedure for introducing Ge into the silicon oxide layer formed belowthe silicon oxynitride layer, before nitriding the surface.

First, as shown in FIGS. 29( a) and 29(b), the silicon substrate 1 dopedwith desired impurities is subjected to diluted HF treatment, so as toterminate the surface of the silicon substrate 1 with hydrogen. Afterthat, the silicon substrate 1 is placed in the chamber of a film formingdevice. The chamber is filled only with a gas (such as a nitrogen gas)that does not react with or etch silicon during the manufacturingprocess. The temperature of the silicon substrate 1 is then heated to700° C., so as to completely detach the hydrogen from the siliconsubstrate 1.

The chamber is then filled with a N₂ gas having a partial pressure of 30Torr and an O₂ gas having a partial pressure of 3 Torr, for example. Thesurface of the silicon substrate 1 is heated to 1050° C., and ismaintained at that temperature for ten seconds. In this manner, asilicon oxide layer is formed on the silicon substrate 1, as shown inFIGS. 29( c) and 29(d). After that, Ge is diffused in the silicon oxidelayer by a technique such as implantation or a heat treatment, so as toform a silicon oxide layer 82 a having a Ge distribution with a peak of1×10¹³ cm⁻² in the silicon oxide layer. The atmosphere in the chamber isthen changed to an O₂ gas having a partial pressure of 3 Torr, forexample, and the surface of the silicon substrate 1 is heated to 750° C.and is maintained at that temperature for ten seconds. By doing so, theGe in the silicon oxide layer 82 a is oxidized, and a Ge level having anenergy level approximately 0.7 eV below the conduction band of SiO₂ isformed in the silicon oxide layer 82 a. Radical nitridation or plasmanitridation is then performed on the surface of the silicon oxide layer82 a. By doing so, a nitrogen-containing layer 82 b is formed on thesurface of the silicon oxide layer 82 a, and 10 atomic % or less ofnitrogen is introduced into the silicon oxide layer 82 a. Through thisnitriding procedure, the hole injecting efficiency in each memory cellis increased. It should be noted here that, in the drawings illustratingthe manufacturing procedures carried out after this procedure, thesilicon oxide layer 82 having the two-layer structure consisting of thesilicon oxide layer 82 a and the nitrogen-containing layer 82 b is shownas a single-layer silicon oxide layer 82.

An amorphous silicon layer of 2 nm in thickness is then deposited on thesilicon oxide layer 82 with the use of a disilane gas. It is preferablethat the temperature of the silicon substrate 1 at this point is 550° C.or lower. The temperature of the silicon substrate 1 is then set at 750°C., and the atmosphere in the chamber is changed to a N₂ gas having apartial pressure of 30 Torr and an O₂ gas having a partial pressure of 3Torr, for example. These conditions are maintained for ten seconds. Inthis manner, a small amount of oxygen is added to the amorphous siliconlayer, and an oxygen-added amorphous silicon layer is formed. Theoxidizing temperature at this point should preferably be 700° C. orhigher, at which the hydrogen in the amorphous silicon layer isdetached. However, the oxidizing temperature cannot be too high, andshould preferably be 800° C. or lower. Furthermore, it is preferable tosupply oxygen while the temperature is rising. Also, the time requiredfor the temperature to rise to a point between 700° C. and 800° C.should preferably be ten seconds or less. Ideally, while oxygen is beingsupplied, the heating-up time is ten seconds or less, and the oxidationtime is ten seconds or less. After that, the atmosphere in the chamberis changed to a N₂ gas having a partial pressure of 30 Torr and a NH₃gas having a partial pressure of 0.03 Torr, for example. The surface ofthe silicon substrate 1 is then set at 750° C., and is maintained atthat temperature for 400 seconds. In this manner, the amorphous siliconlayer is nitrided to form a silicon oxynitride layer 83 (see FIGS. 29(e) and 29(f)). The temperature of the silicon substrate 1 is then set at800° C., and the atmosphere in the chamber is changed to a N₂ gas havinga partial pressure of 30 Torr and an O₂ gas having a partial pressure of0.03 Torr. These conditions are maintained for 100 seconds. As a result,the Si—N—H in the silicon oxynitride layer 83 is oxidized, and thehydrogen radicals are reduced. Thus, a high-quality silicon oxynitridelayer 83 having high reliability can be formed.

The temperature of the silicon substrate 1 is then set at 750° C. orhigher, and a 2.5-nm thick silicon oxide layer 84 is deposited by HTO.By doing so, the tunnel insulating film 85 including the silicon oxidelayer 82, the silicon oxynitride layer 83, and the silicon oxide layer84 is formed as shown in FIGS. 29( e) and 29(f). The temperature of thesilicon substrate 1 is then set at 950° C., and the atmosphere in thechamber is changed to a N₂ gas having a partial pressure of 760 Torr andan O₂ gas having a partial pressure of 3 Torr. These conditions aremaintained for one hour. By doing so, the small amount of hydrogenradicals in the tunnel insulating film 85 is oxidized, and the defectsin the tunnel insulating film 85 are oxygen-terminated. Thus, the tunnelinsulating film 85 can have higher quality and reliability.

A 6-nm thick nitride film to be the charge storage film 86 is thendeposited on the tunnel insulating film 85 by CVD, and a mask material87 for device isolation is deposited by CVD. After that, etching isperformed on the mask material 87, the nitride film (the charge storagefilm) 86, and the tunnel insulating film 85 in this order by RIE withthe use of a resist mask (not shown). Etching is further performed onthe exposed regions of the silicon substrate 1, so as to form deviceisolation grooves 88 of 100 nm in depth, as shown in FIGS. 30( a) and30(b).

A silicon oxide film 89 for device isolation is then deposited on theentire surface, so as to completely fill the device isolation grooves88. After that, the portion of the silicon oxide film 89 on the surfaceis removed by CMP, so that the surface is flattened. At this point, themask material 87 is exposed (see FIGS. 30( c) and 30(d)).

The exposed mask material 87 is selectively removed by etching, and theexposed surface portions of the silicon oxide film 89 are removed byetching with the use of a diluted hydrofluoric acid solution. Afterthat, a 15-nm thick alumina layer 90 a to be the interelectrodeinsulating film is formed on the entire surface by ALD. Due to theoxidizing agent used in the film formation by ALD, a very thin siliconoxide layer 90 b is formed at the interface between the alumina layer 90a and the charge storage film 86. As a result, a 16-nm thickinterelectrode insulating film 90 having a two-layer structureconsisting of the alumina layer 90 a and the silicon oxide layer 90 b isformed (see FIGS. 31( a) and 31(b)).

A 100-nm thick conductive film 91 having a two-layer structureconsisting of a polycrystalline silicon layer and a tungsten silicidelayer is formed to be the control gate electrode in the same manner asin the first embodiment. Further, a mask material 92 for RIE isdeposited by CVD. After that, etching is performed by RIE with the useof a resist mask (not shown) on the mask material 92, the conductivefilm 91, the interelectrode insulating film 90, the charge storage film86, and the tunnel insulating film 85 in this order, so as to form thegate having a stacked structure. In this manner, the shape of the gatehaving the stacked structure is defined, and grooves 94 extending in theword-line direction are formed on the side of this gate having thestacked structure (see FIGS. 31( c) and 31(d)).

As shown in FIGS. 32( a) and 32(b), a silicon oxide film 96 is thenformed on the side faces and the upper face of the gate having thestacked structure by a thermal oxidation technique. After that, thesource and drain regions 97 a and 97 b are formed by an ion injectiontechnique. The region of the silicon substrate 1 located between thesource region 97 a and the drain region 97 b is the channel region 98.Further, an interlayer insulating film 99 is formed to cover the entiresurface by CVD (see FIGS. 32( a) and 32(b)). After that, a wiring layerand the likes are formed by known techniques, so as to complete thenonvolatile semiconductor memory.

Referring now to FIGS. 33 and 34, the effect of adding Ge to the siliconoxide layer 82 is described. FIG. 33 shows the variations in theelectric field required to generate 0.1 A/cm² of leakage current, withrespect to a case where Ge is added to the silicon oxide layer, and acase where Ge is not added. As can be seen from FIG. 33, the inducedelectric field is reduced by 1 MV/cm or more through the Ge addition.This is because, as an electric field is induced, the energy level ofthe Ge level in the SiO₂ layer 82 becomes lower than the Fermi level onthe substrate side, and the electron tunneling probability becomeshigher. In this manner, a high-quality stacked structure that hasexcellent electron injection efficiency and is formed with a SiO₂ layer,a SiON layer, and a SiO₂ layer can be realized by adding Ge to thesilicon oxide layer 82.

The silicon oxynitride layer 83 has a layer thickness of 1.5 nm to 2.5nm, and nitrogen concentration of 20 atomic % to 47 atomic %. In otherwords, 10 atomic % to 30 atomic % of oxygen is contained in the siliconoxynitride layer 83, and at least one of the second nearest neighboratoms of the silicon is nitrogen. This is because oxygen is added to theamorphous silicon layer. In the silicon oxide layer 82, a small amountof nitrogen is diffused when the amorphous silicon layer is nitrided.Therefore, an average of 10 atomic % of nitrogen is contained at amaximum.

In this embodiment, radical nitridation or plasma nitridation isperformed on the surface of the silicon oxide layer. However, to preventnitrogen from penetrating through the silicon oxide layer, it is notnecessary to perform radical nitridation or plasma nitridation, and NH₃or NO may be used instead.

Although an O₂ gas is used when oxygen is added to the amorphous siliconlayer in this embodiment, the gas used at the time of adding oxygen isnot limited to an O₂ gas, and it is possible to use any oxidizing gassuch as NO, N₂O, O₃, O radicals, or O plasma. However, those gasesdiffer from O₂ in oxidation power. Therefore, when one of those gases isused, it is necessary to adjust the oxygen adding conditions in such amanner that the oxygen concentration in the silicon oxynitride layer 83is in the range of 10 atomic % to 30 atomic %.

Although a NH₃ gas is used when the amorphous silicon layer is nitridedin this embodiment, the gas used at the time of nitridation is notlimited to a NH₃ gas, and it is possible to use any nitriding gas suchas NO, N radicals, NH radicals, N₂ radicals, N plasma, NH plasma, or N₂plasma.

Although HTO is used in the process for forming the silicon oxide layer84 in this embodiment, some other technique such as CVD or ALD may beused, as long as the silicon oxide layer 84 can be properly deposited.

In this embodiment, a heat treatment is carried out in an oxidizingatmosphere after the oxide layer is deposited by HTO. This greatlyreduces the hydrogen in the stacked structure formed with a SiO₂ layer,a SiON layer, and a SiO₂ layer. Accordingly, as long as a heat treatmentis carried out in an oxidizing atmosphere after the oxide layer isdeposited by HTO, the temperature at which oxygen is added to theamorphous silicon layer and the temperature at which nitridation isperformed on the amorphous silicon layer may not be very high, and maybe 400° C. or higher.

As described so far, this embodiment can provide a semiconductor devicethat has a high-quality tunnel insulating film in which defects are noteasily formed. This semiconductor device experiences less leakagecurrent.

In the first to fifth embodiments, it is preferable that the amorphoussilicon layer is thinner than 2.5 nm. If the layer thickness of theamorphous silicon layer is 2.5 nm or greater, the following two adverseeffects appear. 1) Sufficient nitridation cannot be performed on theamorphous silicon in the later nitriding procedure, and Si atoms thatare not bonded to nitrogen remains as defects in the film. As a result,the characteristics of the insulating film are degraded. 2) Since the Sidefects are easily oxidized in the later oxidizing procedure, the oxygenconcentration in the oxynitride layer, and the high injection efficiencyas a feature of the insulating film of the SiO₂/SiON/SiO₂ stackedstructure becomes lower.

FIG. 35 shows the C-V characteristics observed in a case where sampleseach formed by depositing an amorphous silicon layer on a SiO₂ layer ona silicon substrate are prepared, oxygen is added to each of the samplesat 750° C. for ten seconds, and nitridation is performed on each of thesamples until nitrogen penetrates through the SiO₂ layer. In a casewhere the layer thickness of the amorphous silicon layer is 2.5 nm, theinterface state is formed as the interface between the SiO₂ layer andthe silicon substrate is nitrided, but hysteresis is not observed in theC-V characteristics. This indicates that the amorphous silicon layer of2.5 nm in layer thickness is nitrided, with only a very small number ofdefects being formed.

In a case where the layer thickness of the amorphous silicon layer is3.5 nm, not only the interface state is formed, but also largehysteresis occurs in the C-V characteristics. The existence ofhysteresis means that a Si structure is locally formed in the nitridelayer (or a Si bandgap exists locally in the bandgap of the nitridelayer), and the Si structure functions as the charge storage source.This indicates that, if the layer thickness of the amorphous siliconlayer is larger than 2.5 nm, nitrogen is diffused into the base layerwhile the amorphous silicon layer remains not completely nitrided.

Accordingly, it is preferable that the layer thickness of the amorphoussilicon layer is smaller than 2.5 nm, as shown in each embodiment of thepresent invention.

In the tunnel insulating film having the SiO₂/SiON/SiO₂ stackedstructure of each embodiment of the present invention, nitrogen showsthe following distribution characteristics. As shown in FIG. 36, thereis a continuous region having nitrogen concentration of 20 atomic % orhigher in a region 5 nm or less from the surface of the SiO₂/SiON/SiO₂stacked structure. This continuous region has a physical film thicknessof 1 nm or more. In the lowermost SiO₂ layer, a nitrogen distributionwith concentration of 10 atomic % or less exists between the lowerinterface and the upper interface. Here, it is preferable that thenitrogen concentration in the lowermost SiO₂ layer is 10 atomic % orless, because the leakage current on the low-and-medium field sideincreases as the nitrogen concentration becomes higher, as shown in FIG.37.

In the first to fifth embodiments, each semiconductor device is anonvolatile semiconductor memory. However, the tunnel insulating filmhaving the three-layer stacked structure formed with a silicon oxidelayer, a silicon oxynitride layer, and a silicon oxide layer may be usedas the gate insulating film of a MOS transistor. The tunnel insulatingfilm may also be used as the interelectrode insulating film in each ofthe first to fifth embodiments. In any of those cases, a layer made of ahigh-k material (such as Hf, La, or a silicate of Hf or La) may be usedin place of the silicon oxide layer as the insulating layer serving asthe base layer on which the amorphous silicon layer to be the siliconoxynitride layer is formed.

As described so far, each embodiment of the present invention canprovide a semiconductor device that has a high-quality insulating filmin which defects are not easily formed, and experiences less leakagecurrent. Each embodiment can also provide a method for manufacturingsuch a semiconductor device.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcepts as defined by the appended claims and their equivalents.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising: forming a first silicon oxide layer on a siliconsubstrate; forming a first silicon oxynitride layer by nitriding thefirst silicon oxide layer; forming an amorphous silicon layer on thefirst silicon oxynitride layer; introducing oxygen into the amorphoussilicon layer; forming a second silicon oxynitride layer by nitridingthe amorphous silicon layer having oxygen introduced thereinto; andforming a second silicon oxide layer on the second silicon oxynitridelayer, thereby forming a three-layer structure including the firstsilicon oxynitride layer having an average nitrogen concentration higherthan 0 atomic % but not higher than 10 atomic %, the second siliconoxynitride layer having an average oxygen concentration in the range of10 atomic % to 30 atomic %, and the second silicon oxide layer.
 2. Themethod according to claim 1, further comprising nitriding a surface ofthe first silicon oxide layer before forming the amorphous silicon layeron the first silicon oxide layer.
 3. The method according to claim 2,further comprising introducing Ge into the first silicon oxide layerbefore nitriding the surface of the first silicon oxide layer.
 4. Themethod according to claim 1, further comprising oxidizing the secondsilicon oxynitride layer before forming the second silicon oxide layer.5. The method according to claim 1, further comprising performingoxidation to the three-layer structure in an oxidizing atmosphere at atemperature between 900° C. and 950° C. for ten minutes or longer, afterforming the three-layer structure including the first silicon oxynitridelayer, the second silicon oxynitride layer, and the second silicon oxidelayer.
 6. The method according to claim 1, wherein the introducingoxygen into the amorphous silicon layer includes introducing oxygen intothe amorphous silicon layer in an atmosphere of an oxidizing gas at atemperature between 700° C. and 800° C. for ten seconds or shorter. 7.The method according to claim 6, wherein the oxidizing gas contains O₂,NO, N₂O, O₃, O radicals, or O plasma.
 8. The method according to claim1, wherein the forming the second silicon oxynitride layer by nitridingthe amorphous silicon layer includes nitriding the amorphous siliconlayer at a temperature between 600° C. and 750° C.
 9. A semiconductordevice comprising an insulating film, the insulating film including: afirst silicon oxynitride layer formed on a silicon substrate and havingaverage nitrogen concentration higher than 0 atomic % but not higherthan 10 atomic %; a second silicon oxynitride layer formed on the firstsilicon oxynitride layer and having average oxygen concentration in therange of 10 atomic % to 30 atomic %; and a silicon oxide layer formed onthe second silicon oxynitride layer.
 10. The device according to claim9, wherein the second silicon oxynitride layer has average nitrogenconcentration in the range of 20 atomic % to 47 atomic %.
 11. The deviceaccording to claim 9, wherein the first silicon oxynitride layercontains Ge.